Northeastern US Continental Shelf SST Report

Temperature tracking for the Northeastern US Continental Shelf Region

Author
Affiliation

Gulf of Maine Research Institute

Published

June 24, 2024

Northeastern US Shelf Region 2024: Sea Surface Temperature

This report was created to track the sea surface temperature regimes for the Northeastern US continental shelf, an area sampled as part of the NMFS Northeast trawl survey.

Satellite sea surface temperature data used was obtained from the National Center for Environmental Information (NCEI). With all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

DISCLAIMER: Any data within 2-weeks of the current date are subject to revision and may change. Please use caution when reporting information that contains these values.

Northeastern US Shelf Region On a Map

Whenever discussing the Northeastern US Shelf Region in this report, we refer to the following spatial extent displayed below. The coordinates of this bounding box are the same coordinates used to clip the sea surface temperature data.

2023: SST in the Northeast Shelf

As these broad regional patterns transpired, the Northeast Shelf was experiencing a range of conditions that stood in contrast to recent years and the timing of the North Atlantic’s record temperatures.

What began as one of the warmest years on record for the Northeast Shelf ended on the cooler side, with seasonal conditions not seen in the last decade.

Figure 2: ?(caption)

Air-Sea Interactions: Extratropical Storm Lee

Observed SSTs may both influence and be influenced by atmospheric weather patterns. These interactions can act to amplify warming or cooling of surface waters. Sustained periods of low cloud cover and the consequent direct sunlight can rapidly warm the ocean at the surface. Steady winds can act as a fan, advecting heat away from the ocean and cooling the surface. Strong winds and storm systems may also lower SST through mixing, which can bring cooler water from depth closer to the surface.

During September, extratropical storm Lee traversed the Northeast Shelf, which weakened from hurricane strength as it crossed over Georges Bank on September 16 ?@fig-lee-traversal. The impacts of this storm on the region’s SSTs can be seen in Figure 2 when temperature fell over 5F over a period of 12 days.

Similar storm crossings have occurred in the recent past, including in 2019 when Hurricane Dorian passed through the Northeast Shelf after working its way up the US East Coast. Both years had some of the fewest autumn heatwave days seen in the past two decades (2019: 0 days, 2023: 6 days).

Comparing Annual SST to Historical Conditions

When we compare the annual average SST in the Northeast Shelf for 2023 (55.27°F) to other years, we can see it narrowly beat out 2012 as the second warmest year on record. When we look at the deviation from the long-term average SST (i.e., the annual SST anomaly), the last decade stands out for its exceptional warmth (Figure 3). 2023 extends a pattern that began in 2010 of sustained above-average temperatures. With the exception of 2019 (ranked 13th), every one of the last ten years remains in the top 10 warmest years on record.

Figure 3: ?(caption)

Average Monthly Conditions

Monthly average SSTs (Table 1.) show how variable conditions were throughout the year. For the first third of the year, monthly SSTs were among the top three warmest for those months. May, June, July, & August average SSTs ranked lower, ranging from 4th to 12th warmest. The final four months were each much cooler, ranging from 12th to 19th warmest for those months.

Table 1:

Average monthly sea surface temperatures in the Northeast Shelf

Table 1. Monthly Average Sea Surface Temperatures -
Month Rank (1982-2023) Observed Temperature Climatological Average Temperature Anomaly

January

4

47.9°F (8.8°C)

45.6°F (7.6°C)

2.3°F (1.3°C)

February

1

46.0°F (7.8°C)

42.8°F (6.0°C)

3.2°F (1.8°C)

March

3

44.4°F (6.9°C)

42.1°F (5.6°C)

2.3°F (1.3°C)

April

2

47.2°F (8.4°C)

44.3°F (6.9°C)

2.9°F (1.6°C)

May

3

51.8°F (11.0°C)

50.1°F (10.0°C)

1.7°F (0.9°C)

June

15

58.6°F (14.8°C)

58.1°F (14.5°C)

0.6°F (0.3°C)

July

2

68.4°F (20.2°C)

65.4°F (18.6°C)

3.0°F (1.6°C)

August

14

68.8°F (20.5°C)

67.8°F (19.9°C)

1.0°F (0.5°C)

September

12

66.0°F (18.9°C)

65.3°F (18.5°C)

0.7°F (0.4°C)

October

19

60.3°F (15.7°C)

60.0°F (15.6°C)

0.3°F (0.1°C)

November

22

54.2°F (12.3°C)

54.3°F (12.4°C)

-0.1°F (-0.0°C)

December

25

48.8°F (9.4°C)

49.7°F (9.8°C)

-0.9°F (-0.5°C)

Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data.
Climatological Reference Period: 1991-2020.

Daily SSTs in the Northeast Shelf

The annual cycle of SST in the Northeast Shelf exhibits a pattern common to regions in the Northern Hemisphere, with the lowest temperatures observed in March and the highest values observed in August (Figure 4). Daily SST anomalies in 2023 never fell below +-1.97 °F compared to the long-term (1991-2020) average and reached as high as high as 4 °F above the long-term average.

The largest temperature anomalies were observed during November, which also turned out to be the warmest November on record (as highlighted in our Fall 2022 update). Figure 4 also illustrates that, in 2023, 123 days experienced SST anomalies that exceeded the threshold for being considered marine heatwaves, or MHW (more on MHWs below, including Figure 8)

Figure 4: A timeseries of daily SSTs (January 1 through December 31, 2023) with an indication of marine heatwave (MHW) conditions in the Northeast Shelf. Black lines represent the long-term (i.e., 1991 – 2020) average SST, the 10th percentile (i.e., cold spell threshold), and 90th percentile (i.e., heatwave threshold). A solid line indicates the observed SST for each day; red and blue shading illustrates whether each day is considered part of a MHW event.

SST Anomalis & MHW Events

A marine heatwave (MHW) is defined as a period when there are five or more consecutive days when the observed SST is greater than the 90th percentile of the long-term average for that day. Gaps of 2 days or less in this threshold do not constitute a break in the MHW event. Figure 4 illustrates that the Northeast Shelf met the criteria for experiencing a MHW for 123 days in 2023 (or 97% of the year). Superimposing MHW status (black line) over the full timeseries of daily SST anomalies (blue/white/red shading) (Figure 4) reveals that the frequency, duration, and intensity of MHWs has increased in the past decade.

Figure 5: A timeseries of marine heatwave (MHW) conditions in the Northeast Shelf extending from January 1, 2023 through December 31, 2023. The black line represents the long-term (i.e., 1991 – 2020) average SST. The 10th to 90th percentile range is shaded in gray. The heavy line (red for marine heatwave or blue for a non-event) indicates the observed SST this year.

Record Setting Daily Sea Surface Temperatures

Daily SST values in 2023 were some of the highest ever recorded in the Northeast Shelf. Record high SSTs were set for more than half of all days during the months of February, March, & April. February saw the most new records, with 86% of days setting new daily record highs.

Figure 6: ?(caption)

Decline of Below Average SST Temperature Days

As SST in the Northeast Shelf continues to rise, the fraction of days when the region experiences below-average temperatures is expected to continue to fall. In a world without human-caused climate change, we would expect, positive (warm) and negative (cool) SST anomalies to more or less balance out over the span of several years, as various patterns of natural climate variability alternate having a dominant influence on Earth’s climate (e.g., La Niña vs El Niño). What is being observed in the Northeast Shelf (and elsewhere around the world), however, is a loss of that balance: larger fractions of recent years are experiencing above average temperatures and cold spells are becoming vanishingly rare.

During 2023 the Northeast Shelf experienced SSTs below the long-term average for a total of 48 days. A period of less than 2 months, and with no days experiencing temperatures that would qualify as marine cold-spells, the cool-water equivalent of a MHW. These events have become rare, with just 5 days of cold spell conditions in the last 20 years.

Figure 7: ?(caption)

Northeast Shelf SSTs Out of Balance

The complete picture of above and below average conditions for the Northeast Shelf reveals a region out of balance. Since 2010 the Northeast Shelf has experienced a disproportionate number of anomalously hot SSTs and experienced many prolonged MHW events.

Figure 8: ?(caption)

Annual SST Anomaly Map

In addition to the sub-annual (or temporal) variation, there can also be significant spatial variability in annually-averaged SST patterns (Figure 10).

This year the impacts of large-scale variability in major oceanic currents, such as the relative influence of the Gulf Stream versus the Labrador Current was quite apparent. With large pockets of above and below average SST anomalies present along the turbulent path of the Gulf Stream to the South of our study area.

Figure 10: ?(caption)

Quarterly Average SST Maps

Looking at these spatial patterns across the annual quarters of 2023 (Figure 11) shows not only the large spatial variability in SST conditions, but also the significant variation in those patterns over the course of the year.

Figure 11: ?(caption)

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A work by Adam A. Kemberling

Akemberling@gmri.org

 

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